Coupling systems for permitting a number of receiver/transmitter units to communicate along a common data bus are well known in the art. For example, in telephony hybrid transformers are used along a telephone systems cable lines in a two-wire communication system for providing directional coupling between telephone sets. The hybrid transformers typically used in telephony provide bi-directional communication if two terminals in the communication system are connected at the far ends of a common cable. If a third terminal is connected to the communication line between the other two terminals at the extremes of the cable, and the other two terminals are each simultaneously transmitting signals down the line in opposite directions, the third terminal will receive a signal that is a mixture of the signals being transmitted by the other two terminals, and as a result will be unable to decipher or receive information from either of the other two terminals unless the signals are time displaced, phase displaced, or in some other way made electrically different to permit detection by a third terminal from the common transmission line. The typical telephony hybrid couplers must have a balanced network to operate in proper termination, whereby if along a common telephone cable an open circuit or short circuit occurs, the telephony hybrid couplers cannot provide communication for terminals connected to the cable system upstream of the failure point in the cable.
Directional couplers are also known in the art for use in microwave transmission systems. However, microwave directional couplers by necessity use wave guide techniques that are not practical in the frequency range of 100 KHz to 10 MHz, where coaxial cables or twisted shielded cables serve as the transmission media or data bus for data signals.
Also, it is known to use data bus systems as defined in MIL-STD-1553B in aerospace instrumentation systems for digital data transmissions between units on a common data bus. Such systems provide for protection against short circuits occurring in remote units or transformers connected to the bus, no protection is provided against cable termination failures, whereby an open or short circuit failure anywhere along the main cable bus causes the entire bus system to fail.